The fact that after intensive illumination (so-called photo-stress) a certain period of time is needed for the visual functions of the eye to return to the level before the illumination is well known from the art. The biological explanation to the phenomenon is that light induces a change in the chemical structure of the visual purple (the photoperceptive material of the eyes) and it temporarily loses its photoperceptive ability. Under normal circumstances the original light sensitivity of the visual purple returns in a fraction of a second. However, as a result of intensive illumination and in the case of certain diseases this recovery time is significantly longer (Magdar J. Am. J. Ophthalmology, 1962, 49:147-150; Forsius H., Eriksson AW, Krause U. Acta Ophthalmologica, 1963, 41: 55-63; Glaser JS, Savino PJ, Sumers KD, McDonald SA, Krighton RW Am. J. Ophthalmology, 1977, 83: 255-260). Diseases resulting in the disturbance of the regeneration of the visual purple and the lengthening of the recovery time of the visual functions are rather common. Such diseases include first of all the senile degeneration of the central part of the retina, retinopathy associated with serious short-sightedness, diabetes and vascular sclerosis. A large number of other eye diseases and the lasting use of certain drugs may also cause visual complaints. Presently not much attention is paid to solving this problem, first of all because no appropriate apparatus is available.
Optokinetic nystagmus is a physiological phenomenon known for decades (Catford, G.V., Oliver, A. Proo 2nd International Orthoptic Congress, Excerpta Medica Amsterdam, 1971). Essentially it means that the eyes involuntarily, automatically follow the movement of objects or images perceived in their visual field. If the movement is periodic, the eyeballs also move there and back with the same frequency as the perceived object (and this pendular oscillation of the eye is called nystagmus). Or the other way round, if the eye moves, it sees the moving object, but if it stays still, then it does not.
Another known phenomenon is that they eye can perceive the flashes of a vibrating light source separately only to a certain frequency. If the frequency of the flashes is higher than this, the eye perceives the vibrating light signals merged, as constant light. This frequency is called critical fusion frequency (CFF).
In the laser surgery of the cornea an instrument called “eye-tracker” is used to follow the position of the eye during the operation. This instrument perceives the position of the eye and controls a laser knife. The laser knife works as long as the eye is motionless, and it stops as soon as the eye moves away from its position called mid-position.
The adaptometer is a known ophthalmologic diagnostic instrument used for measuring the dark adaptation of the eye. When using it, after staying in complete darkness for ten minutes three minutes of continuous illumination is used with white light. The amount of illumination is 2,000-3,000 lux. After this an image of certain brightness just not visible to the examined person occurs periodically in front of the eye. When after a certain period of time the person perceives the picture, another image occurs, which is illuminated less by one logarithmic grade. After some more time the examined person will also recognise this image; then a new image appears, which is illuminated at an even weaker extent. It goes on like this for 45 minutes. The extent of adaptation is the sign or image illuminated at the weakest extent, which the, eye can still recognise in the course of the 45-minute examination (Goldman, M.H.: Un nouvel adaptometre automatique. Bull Soc. Franc Optalmol 1950, 63:4-17). This apparatus is not sensitive enough, it is suitable to indicate only rough differences, Besides the examination is very lengthy, especially if the two eyes are examined separately. It is not a common examination of the everyday practice.
The nyctometer is a known device used for determining the light adaptation of the eye (Hartman E, Wehmeyer K.: Klinische Monatsblatter für Augenheilkunde, 1980, 176:859-863). In this case test images of the same size are used, but they are illuminated in a different way as compared to the background. Eight different variations can be used, where the proportions of the illumination of the background and the image are the following: 1:23, 5; 1:4, 87; 1:2, 71; 1:2, 00; 1:1, 66; 1:1, 46; 1:1, 25; 1:1, 14. The aim of the examination is to determine the image illuminated at the least extent, which the examined person can still recognise. This apparatus has another version with a built-in clock; and it is also suitable for measuring the time needed to recognise the image illuminated at the least extent. This apparataus is not suitable for the objective determination of visual functions, and it has a rather restricted indication field, and for these reasons it is not widely used in practice.
The object of the invention was to create a new process and apparatus, which, by means of examining the visual functions of the eye under standardised.: circumstances, make it possible to recognise the early signs of visual damage of ophthalmologic and non-ophthalmologic origin.